This invention relates to a high frequency heating apparatus in which an input power supply is converted by a frequency converter to a high-frequency electric power, which is supplied via a step-up transformer into a magnetron to drive the same, and more particularly to such a high frequency heating apparatus which can be supplied with electric power not only from a commercial power supply but other power supplies such as an automobile battery.
In high frequency heating apparatus which have been commercially produced, an electric power from a commercial power supply is supplied to an inverter or frequency converter to be converted to a high frequency power, which power is supplied to a magnetron through a step-up transformer so that the magnetron is driven for cooking food by way of high frequency heating, as well known in the art. In such a conventional high frequency heating apparatus, the magnitude of the electric wave or high frequency output depends upon the magnitude of a magnetron anode current which further depends upon an "on" period of a frequency converter switching element for the frequency conversion. Accordingly, in many cases, the "on" period of the frequency conversion switching element is determined in accordance with one commercial power supply voltage such that the anode current is maintained at a predetermined value. Levels of the heating intensity are adapted to be changed by driving the magnetron continuously or intermittently without controlling the magnitude of the anode current.
When the conventional high frequency heating apparatus employing the above-described arrangement for determining the high frequency output and the heating intensity is used in a region where a rated voltage of the commercial power supply differs from that determined for the apparatus or when the rated voltage of the commercial power supplies provided differs from room to room, the magnetron may not be normally operated because of a low input voltage or to the contrary, an abnormal voltage may be applied to the magnetron because of a high input voltage. Furthermore, it is almost impossible to operate the high frequency heating apparatus when the same is arranged to be supplied with the electric power from an automobile battery for an outdoor dinner. That is, it is almost impossible to use the conventional high frequency heating apparatus from place to place easily according to different power supplies provided.